Table 1 lists target performance and main specifications of the prototype engine. Their values were determined based on discussion of heat exchangers and a piston drive mechanism with the calculated results of the above method. Cylinder bore, piston stroke and target power were determined to be 36 mm, 10 mm and 50 W/4000 rprm respectively. In the case of a compact engine, high efficiency is not expected, because the rate of heat conduction loss increases relatively with the reduction of the engine size. Target thermal efficiency is 15%. This value is much lower than that of high performance Stirling engines mentioned previously.
Allowable pressure in the engine was determined to be 1.1 MPa with consideration of using piping parts in markets. Allowable heater wall temperature was determined based on strength and thickness of heater wall material. In this case, stainless steel (SUS304) was used as the material, and a heater was designed considering allowable heater wall temperature of 650〜800℃.
2.2 BASIC STRUCTURE
A basic structure of the prototype engine was based on that of a 100 W class Stirling engine formerly developed by the author and others [3], [4]. Figure 2 shows a schematic view of the prototype engine. It was a gamma type, in which a displacer and a power piston were located in a line. A regenerator was located in the displacer. These configurations contribute to make the engine smaller.
In order to get a smaller size and low production cost, a simple moving-tube-type heat exchanger was adopted as shown in Fig.3. In the case of a moving tube type heat exchanger of the above 100 W class Stirling engine, there were 10 - 24 heating tubes to get large heating surface area. However, the tubes cause high production cost. Also, high assembling accuracy is required to avoid the collision of the inner and outer tubes. From the above Considerations, in the case of the prototype engine, only a couple of inner-outer tube was located in the central axis of the displacer. The inner tube of heater, and the inner and outer tube of cooler were easily assembled. Therefore, it is considered that this type of heat exchanger obtains pretty lower production cost than that of the above 100 W class Stirling engine and previous multi-tube type heat exchangers. In addition, the new type heat exchanger needs little welding.
As a piston drive mechanism, it was considered to adopt a cross-head mechanism, a Scotch-yoke mechanism and other link mechanisms. Finally, a Rhombic mechanism [5] was adopted, because it gives an excellent momentum balance.
Fig.2 Schematic view of the prototype engine
Fig.3 Heat exchanger of the prototype engine
2.3 COMPONENTS DESIGN
(1) Heater
A heater of the prototype engine was designed using the following simple method. Heater input, Qh, was derived from Eqs. (1) and (2).
Qh + LE + Qr + Qcd (1)
Qh = hS(Tw - TE) (2)
Here, LE is expansion power calculated from Schmidt theory [6], Qr is reheat loss in a regenerator, Qcd is heat conduction loss at a cylinder, h is heat transfer coefficient in a tube, S is heating surface area, TE is expansion space gas temperature. Heater wall temperature, Tw, was gotten by solving simultaneous equations for Qh in Eqs. (1) and (2). Gas flow speed needed in this calculation was based on mean piston speed of a displacer. General design method for a multi-tube type heat exchanger mentioned previously was also applied to this engine, using hydraulic diameter of annular space of a moving-tube-type heat exchanger.
Figure 4 shows one of the calculated results of the relationship between mean pressure, Pm, and heater wall temperature, Tw. Here, CL is clearance between an inner tube and an outer tube. This figure shows that the heater wall temperature, Tw, of 660℃ is needed to get expansion space gas temperature, TE, of 600℃, at CL = 0.5 mm, and Pm = 0.8 MPa. Also, heat transfer performance became higher with decreasing of clearance, CL. In order to avoid a collision between the inner and outer tube, it is necessary that the clearance, CL is determined with a consideration of assembling accuracy of a piston drive mechanism. Two types of heater were manufactured for the prototype engine.
Fig.4 Calculated result as a function of mean pressure and heater wall temperature
